ISSN 0030-400X, Optics and Spectroscopy, 2014, Vol. 116, No. 3, pp. 379–383. © Pleiades Publishing, Ltd., 2014. Original Russian Text © A.S. Shcheulin, A.E. Angervaks, A.V. Veniaminov, V.V. Zakharov, A.I. Ryskin, 2014, published in Optika i Spektroskopiya, 2014, Vol. 116, No. 3, pp. 408– 412. 379 Recording of holograms in ionic crystals with color centers occurs according to the diffusion-drift mecha- nism, according to which the centers undergo trans- formation from one type into another and spatial redistribution [1, 2]. The essence of this mechanism is as follows: the radiation that is used to record a holo- gram causes photoionization of color centers, which are associates of anionic vacancies and electrons; their number is the same as in the neutral state. This num- ber varies from unity (F centers) to several tens and even hundreds of thousands (highly aggregated cen- ters). The electrons formed under these conditions diffuse from maxima of the interference field to its minima and are captured by traps (i.e., color centers) [3]. As a result, an electric field is induced between the maxima and minima. Anionic vacancies, being split from an ionized center due to the thermal effect (holo- grams are recorded in these crystals at a rather high temperature) or formed due to its ionization (if the case in point is an F center), drift under the field toward interference minima. Here, they recombine with the electrons released as a result of thermal ion- ization of traps and form color centers. Thus, color centers move from maxima to minima of the interfer- ence field in which a hologram is recorded, although the real motion is performed by their components. This process can be accompanied by a change in the center type, because the conditions under which color centers are formed in minima differ from the condi- tions corresponding to the formation of the “initial” (i.e., photoionized in maxima) centers. The diffusion-drift mechanism of hologram recording proposed for alkali-halide crystals with color centers in [1, 2] was used to describe the record- ing of holograms on color centers in CaF 2 (fluorite) crystals in [4]. In this paper, we report the results of studying the mechanisms of transformation of color centers during hologram recording in CaF 2 crystals. To form color centers in a CaF 2 crystal, the latter was subjected to additive coloring (heating in cation metal vapor [5–7]). As a result, anionic vacancies and electrons with a concentration of ~10 17 cm –3 were introduced into the crystal. “Simple” color centers (F, M, R, and N) are generally formed at this (relatively low) concentration. They are composed of one to four anionic vacancies, respectively, with the same num- bers of electrons and a small number of the so-called “colloidal” centers, which are highly aggregated; these are two-dimensional calcium inclusions in the fluorite lattice [8, 9]. A plate 10 × 10 × 1.2 mm in size was cut from a colored crystal. Its absorption spectrum, mea- sured with a Cary 500 spectrophotometer, is shown in Fig. 1. Colloidal centers manifest themselves in the form of a weak shoulder near ~600 nm; other absorp- tion bands are due to simple centers. A hologram was recorded in the sample by 532-nm radiation (a Cobolt Samba laser with an output power of 500 mW) at a temperature of 190°С in a thermo- stated interferometer, according to a symmetric trans- mission scheme with a convergence angle of 2.7° on a 10 × 10-mm face; this angle value provided a holo- graphic-grating period of 11 μm. The contrast of a sinusoidal interference field recorded in the form of a hologram was no less than 99%. The exposure during recording was 31 kJ/cm 2 . The absorption spectrum of the sample with a recorded hologram is also shown in Fig. 1. One can see that the recording leads to a significant increase in the CONDENSED-MATTER SPECTROSCOPY Transformation of Color Centers during Hologram Recording in an Additively Colored CaF 2 Crystal A. S. Shcheulin, A. E. Angervaks, A. V. Veniaminov, V. V. Zakharov, and A. I. Ryskin University ITMO, 197101 Russia e-mail: angervax@mail.ru Received September 17, 2013 Abstract—When holograms are recorded on color centers in calcium fluoride crystals, these centers undergo spatial redistribution in the crystal bulk, which is accompanied by their transformation. The nature of this transformation has been investigated by optical spectroscopy and confocal scanning microscopy. It is shown that, under the recording conditions we used, the degree of center aggregation increases in both minima and maxima of the interference field in which the recording performed. The enhanced aggregation in field min- ima is caused by the increase in the concentration color centers, while the additional aggregation in maxima is determined by the specific conditions of hologram recording: the wavelength and power density of record- ing radiation and the crystal temperature. DOI: 10.1134/S0030400X14030187